The present disclosure relates in general to fabrication of micro electro-mechanical systems (MEMS) devices, and more particularly, to patterning alignment marks on a transparent substrate in the fabrication of MEMS devices. Still more particularly, the present disclosure relates to a method and system for patterning alignment marks on a transparent substrate for a substrate spatial light modulator having a plurality of reflective elements.
Deformable-mirror spatial light modulators (also referred to as digital light processors, deformable mirror devices, or DMDs) typically include a plurality of movable elements, such as reflective elements or digital micro-mirrors. In a common application, the digital micro-mirror is coordinated with a digital video or graphic signal, a light source, and/or a projection lens to reflect a digital image onto a subject. Conventionally, the plurality of reflective elements are built on a single substrate such a an optically opaque silicon substrate, which also further includes one or more CMOS control circuits. The fabrication process of these conventional single substrate structures is complicated, costly, and often produces a low yield.
A double substrate spatial light modulator may be used to fabricate one or more MEMS devices. The double substrate spatial light modulator typically comprises a plurality of reflective elements, each having a front surface that faces an optically transparent substrate and a back surface that faces an optically opaque substrate, for example, a semiconductor silicon substrate. The reflective elements may be selectively deflected or twisted to spatially modulate light incident to the upper substrate (optically transparent substrate) to reflect light back to the upper substrate. The twist angle of the reflective elements may be controlled by adjusting an input voltage of the driving device. Thus, different reflection paths may be treated as on/off states.
The fabrication process of the double substrate spatial light modulator includes forming an addressing circuitry and electrodes on a semiconductor substrate. An attraction electrode is also attached to the back surface of the reflective elements. When a voltage bias is applied between an attraction electrode on a reflective element and a corresponding actuating electrode on the semiconductor substrate, the attraction electrode is pulled towards the actuating electrode, which causes the reflective element to deflect. Typically, an increased tilt angle of the reflective elements improves the contrast ratio and gray scale of the mirror projector.
In addition to electrodes, the fabrication process of the double substrate spatial light modulator includes aligning and joining of the optically transparent substrate and the semiconductor substrate. The contact areas on the optically transparent substrate and the semiconductor substrate must be patterned in the wafer level fabrication process. However, when the two substrates are joined together, blind stepping, which ignores wafer alignment marks, causes a pattern definition deviation on the optically transparent substrate. As a voltage bias is applied, the tilt angle of the reflective element motion is decreased, which degrades the contrast ratio and gray scale of the mirror projector. In addition, when an alignment laser light passes through the deposition layer, such as silicon dioxide or indium tin oxide (ITO) of the transparent substrate, for patterning, the light penetrates the optically transparent substrate and weakens the reflective light intensity detected by a photo detector. As a result, the resulting products are often defective.
Therefore, a need exists for a method and a system that accurately patterns alignment marks on a transparent substrate, such that the joining shift problem of two substrates may be reduced or eliminated. In addition, a need exists for a method and a system that accurately patterns alignment marks on a plurality of dies on the transparent substrate, such that wafer rejection by the alignment apparatus may be minimized.